Condensation vacuum pump



April 8, 1941. F. E. BANCROFT ETAL 2.237.806

CONDENSATION VACUUM PUMP Filed Jan. 11, 1940 Inventovs: Frank E.Bahc\-o1t, Charles W. Miller,

Th ei-r- Attorn e a circle, there is for any given radius of arc, a maximum free path for a molecule in the space in the plane of the arc. Now it can be seen that the smaller the radiusof the arc, the smaller is this maximum'free path. The existence of long free paths introduces the possibility of collisions between vapour molecules such as may cause some of the molecules to reach the high vacuum or fine side of the jet so that back streaming may be set up giving rise to back diffusion. This back diffusion besides having the wellknown effect of lowering the speed of the jet, interferes with the mass downward movement of the vapour to cause a lowering of the critical backing pressure of the jet.

It will thus be clear that, from consideration of the above effect only, it is desirable to keep the radius of the annular gap small in a jet designed for relatively high backing pressure.

'According to the present invention which is based upon the considerations set forth above, in a vertical pump of the kind specified the cowls of some or all of the jets other than the uppermost jet are substantially not'of greater diam eter than that of the uppermost cowl and conveniently these cowls have approximately the same diameter as that of the-uppermost cowl, whilst in addition the bore of the pump casing or surrounding tubular condenser surface is made less, preferably progressively, where it co-operates with the lowermost and any intermediate jets, than is the case with the uppermost jet of the pump. Such reduction in diameter can be effected in various ways of which several are hereinafter exemplified.

After considerable experiment and adjustments of the various parts and operating conditions it has been found that better results are obtained with a pump according to the invention above set forth, if the several jets are fed from respective uptake tubes instead of from a single common uptake tube and a possiblereason for this is that since with a single uptake tube the amount of vapour being fed from any one jet is dependent to some extent on the vapour being fed from another jet and since the several jets feed into such relatively widely different forevacua, variation of vapour distribution occurs when the final backing pressure, that is that of the lowermost jet is allowed to increase.

Therefore, according to a subsidiary feature of the invention the several jets of the pump are fed from respective uptake tubes or they might be fed from separate boilers although the latter alternative is obviously less practicable. It is desirable, however, that the jets should act as though fed from separate boilers. Multi-jet pumps having separate uptake tubes have, of course, already been proposed, originally, possibly in British Specification No, 251,213 describing a multistage mercury vapour pump.

It is not necessary that the several uptake tubes should be carried to the bottom of the common boiler although obviously they must be carried below the oil level, or below the respective oil levels which obtain during the operation of the pump.

According to a still further subsidiary feature of the invention the diameters of those parts of the uptake tubes which extend into the boiler are varied so as to adjust the effective boiler area which feeds each jet since in general it has been found desirable for good results that the boiler area feeding the lower jets should be greater than that feeding the higher jets. As

the result of experiments, one particular pump which has been constructed having two stages and which had good characteristics, the ratio of the boiler area feeding the lower jet was twice that feeding the upper jet.

In carrying out the main feature of the invention the pump bore or condenser surface may be made conical so that the gaps between the several jets and the condenser surface are graded in the manner hereinbefore indicated, the

several cowls of the jets being conveniently of substantially the same diameter and associated with respective uptake tubes which at their lower ends are flared or expanded and extend into the boiler, which latter is of wider diameter than that of the smallest internal part of the pump casing.

In another arrangement the condenser surface or pump bore may comprise a plurality of cylindrical portions of graded diameter and merging the one into the other by short conical portions which may be located above the respective associated jets.

In a further alternative arrangement of carrying out the invention a pump substantially as illustrated in the specification of British Patent No. 346,293 may be modified in the following ways. Firstly, a second cowl of annular form is secured around the uptake tube 2 shown and described in the'aforesaid earlier specification, this additional cowl being preferably of cylindrical shape. The external cylindrical diameter of this cowl is preferably substantially the same as the maximum diameter of the uppermost cowl 6. Within the confines of this second cowl the uptake tube 2 is provided all around it with a plurality of perforations and above these is a ring which is externally secured to the inside of the uptake tube 2 of the aforesaid earlier specification, and internally has secured to it the upper end of a second uptake tube which also has an expanded lower end dipping into the boiler. With this arrangement it will be appreciated that for the additional jet,

vapour from the boiler travels in the annular space between the original and the additional or inner uptake tubes and emerges from the aforesaid perforations.

Secondly, the main feature of the present invention is completed in this arrangement by the provision within the uniform bore of the condenser or pump body, of a tubular member or liner which, as will be appreciated, effects the desired reduction of the pump bore for co-operation with the lower cowl to form the jet of the relative dimensions hereinbefore set forth. Said liner should either be itself cooled by the circulation of a cooling fluid therein for which purpose it should be channelled or made hollow, or as has been found, it can be maintained sufficiently cool if its external cylindrical surface is made a good push fit within the bore of the pump body or condenser surface that is to say if care is taken that a good thermal contact is made between these members.

The upper and lower ends of the liner tube are preferably flared from the interior. Preferably also the upper and lower ends of the liner are externally reduced slightly in diameter so as to provide oil traps which effectively prevent air from being sucked up between the liner and the pump bore.

It. will be understood that a still further jet might be added by making the bore of the aforesaid liner member stepped or conical and such 4 with and provided with a few an arrangement is 01' general application for the purpose oi the invention.

The original pump bei'ore modification as aforesaid, has a speed or litres per second and a backing pressure of about 80 when operating with what is sold as "A-piezon B oil as the workoperating with 600 watts in the heater. After modification, under the same conditions, the backing pressure was increased about times. It will be appreciated that still higher be quite so 800d.

According to still another subsidiary feature of the invention the pump may be arranged to fractionate in the manner which is per se well known and has been described by various workers in technical publications, whilst as a convenient ex-- ample of a pump adapted to fractionate the specification of British Patent No. 475,062 may be referred to. To this end the uptake tubes may be brought down close to the surface or the boilers;

into contact thereholes in relatively preferably they are brought staggered arrangement so as to ensure that the of time in doing so.

In the accompanying, drawing-*- Figure 1 is adiagrammatic elevation showing how tubes 5, v the diflusion pump. At condenser surface which ing jacket 9. The jets in a manner per se well tive cowls III, II and I2 ends of the uptake tubes 2, I and l are provided known with the respecat the respective upper ii, 8 and .1. The lower creases downwards.

Referring to Fig. 2 of the ing, reference should be accompanying drawmade to the drawing being conical whilst includes a plurality of superimposed cowled Jets upper cowl 215 which accompanying the tens Patent No. 346,293 aforesaid. Comparing V is supported on the central rod 21 of high thermal conductivity material, from the floor of the boiler 20. It will be seen that the lower ends of the uptake tubes are brought down to the floor of the boiler 20 and are provided with perforations 28 and 29 whereby the pump may be self-fractionating. In this respect it will be appreciated that as indicated the level of the oil in the three annular c0m partments is different during operation.

Referring lastly to Fig. 3 01' the accompanying drawing. which shows only a boiler and three uptake tubes for a three-jet pump, the tubes are represented at 30, 3| and 32, whilst the boiler is indicated at 33. The tubes are flared and brought down to the floor of the boiler whilst holes 34, 35 and 36, few in nlunber and 1. A vertical diffusion pump of the type which discharging downwardly into a condensing the wall of the pumping champurpose and with the efl'ect of decreasing the clearances between the lower cowls and the said wall.

2. A din'usion pump according to claim 1 in which the lower cowls are of cylindrical contour and uniform diameter, Ie the pump wall is of downwardly tapered character. 

